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However, in terms of insert performance, if the storage subsystem contains many spindles and you’re loading data from multiple sessions, the random order can actually be better than sequential despite the splits.

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Cover of Training Kit (Exam 70-461): Querying Microsoft® SQL Server® 2012

Note

Spindles are the "poles" that hold platters in the disk, with each platter being reached by an access arm with a read/write head. More spindles mean data is stored on more platters, meaning it can be accessed simultaneously by more user sessions with less contention (or waiting time between jobs). The same goes for input, random data across spindles can be inserted by more heads and operate faster versus sequential data loading one platter until full.